This invention relates to rotary electric machines and more particularly to self-cascaded alternating current reluctance motors having axially laminated rotors.
Self-cascaded rotary machines include a stator winding which is wound to produce two fields of different pole numbers. Such machines have been designed which utilize squirrel cage rotors, wound rotors or reluctance type rotors. U.S. Pat. No. 3,686,553 issued Aug. 22, 1972 to Broadway et al, discloses a self-cascaded three-phase alternating current machine having a stator winding and a rotor wherein the stator winding has component coils connected between two sets of terminals and is wound to provide a winding of a first pole number between one set of terminals and a second pole number between the second set of terminals with the rotor being constructed such that when it is rotated relative to a magnetic field of the first pole number, it creates a magnetic field of the second pole number which rotates in the opposite direction of the first field relative to the rotor. The rotor may be of the wound type, the reluctance type, or a combination of both the reluctance type and wound forms. Self-cascaded electrical motors can be made to run synchronously by the simultaneous application of alternating current and direct current to the stator windings.
Reluctance motor rotors have been constructed by assembling a plurality of radial laminations which are stacked axially. Openings were provided in the radial laminations to control the ratio of the direct axis reactance x.sub.d to the quadrature axis reactance x.sub.q. Since improved synchronous performance can be achieved by increasing the x.sub.d /x.sub.q ratio, axially laminated rotors were developed to provide a larger x.sub.d /x.sub.q ratio. Reluctance motors using axially laminated rotors having been disclosed by Cruickshank et al. in "Axially Laminated Anisotropic Rotors for Reluctance Motors", Proc. IEE, Vol. 113, No. 12, pp. 2058-2060, December 1966, and "Theory and Performance of Reluctance Motors with Axially Laminated Anisotropic Rotors," Proc. IEE, Vol. 118, No. 7, pp. 887-894, July 1971. These articles disclose four pole rotors of cut C core construction. The rotors are assembled by winding strips of cold-rolled, grain-oriented steel on circular forms using standard C core winding methods. Four C cores are cut from this winding, inverted and bolts to a steel shaft using non-magnetic bolts. The rotor is then machined to the appropriate diameter. Rotors having a pole-pitch to pole-arc ratio (B) of 0.94 have been constructed in this manner. However, it has been found necessary to limit the value of B to approximately 0.45 to allow sufficient room for cage bars, which improve the motor starting characteristics. Rotors constructed using standard C core methods as described have limited B values, a limited number of poles, relatively complex construction, high rotor inertia, and the number of rotor poles is practically limited to four.
U.S. Pat. No. 4,110,646 issued Aug. 29, 1978 to Rao discloses a synchronous reluctance motor with an axially laminated rotor having an even number of segments extending from one pole center to another with each segment composed of a plurality of interleaved magnetic and conductive sub-segments to provide increased direct axis reluctance and decreased quadrature axis reluctance. The present invention discloses a doubly-excited reluctance motor having an axially laminated rotor which can have either an even or odd number of poles, wherein the number of rotor poles is equal to the number of pole pairs between two sets of stator winding terminals, or an integral submultiple of that number, with the stator being wound such that the number of poles between one pair of terminals differs from the number of poles between the other pair of terminals by more than two.
By using an axially laminated rotor having a large direct axis to quadrature axis reluctance ratio and applying a direct current through part of the stator winding, a self-cascaded synchronous reluctance motor having a leading power factor can be constructed in accordance with the present invention. Variable speed operation can be achieved by driving the motor with a variable frequency inverter power source. Since the motor has a leading power factor, a naturally commutated inverter can be used which is less expensive to design and build than a forced commutated inverter. In addition, since the mechanical output power in a cumulative cascade connection is provided by the two stator component windings in a ratio approximately equal to that of their pole numbers, it is possible to choose a proper pole combination such that less power is provided through the a.c. component winding. This will reduce the size, cost and weight of the naturally commutated inverter.